Influenza virus activity in the world

6 January 2012

Source: Laboratory confirmed data from the Global Influenza Surveillance and Response System (GISRS).

Virological summary

During weeks 49 to 51 of 2011 (4 to 24 December 2011), laboratory confirmed influenza activity started increasing slightly in some countries in the northern hemisphere. In general influenza activity remained low.

Globally influenza A(H3N2) was the predominant virus subtype detected. Influenza A(H1N1)pdm09 detection was very low, while the proportion of circulating B virus varied.

In Europe, North America and the Middle East, an increase of influenza A(H3N2) activity was observed in some countries with localized to regional activity reported. Influenza B virus was detected at low levels with A(H1N1)pdm09 detected to a lesser extent.

In Asia, activity of influenza viruses in various proportions started increasing in some countries at local to regional levels. Influenza A(H3N2) virus predominated in Japan and the Republic of Korea, while influenza B predominated in Cambodia, China and Singapore. In some other countries, A(H3N2) and B co-circulated. Influenza A(H1N1)pdm09 detection was low.

In the southern hemisphere, at low levels, among circulating influenza viruses, A(H3N2) predominated in general.

A human case of influenza A(H5N1) virus infection was reported in China. The virus belongs to clade 2.3.2.1, from which, two candidate vaccine viruses are being developed by the WHO GISRS. Virus characterization is on-going in GISRS laboratories in Beijing and Hong Kong.

An additional human case of infection with a variant influenza A(H3N2) virus A(H3N2)v was confirmed in the United States of America. The A(H3N2)v virus has 7 genes from the triple reassortant A(H3N2) viruses known to have been circulating in pigs in North America and the M gene from an A(H1N1)pdm09 virus. This is the 12th human case of A(H3N2)v infection. All these viruses characterized so far are antigenically closely related to the candidate vaccine virus developed from A/Minnesota/11/2010.

FluNet summary

During weeks 49 to 51 of 2011 (4 to 24 December 2011), National Influenza Centres (NICs) and other national influenza laboratories from 90 countries, areas or territories reported data to FluNet. The WHO GISRS laboratories tested more than 41995 specimens. 2433 were positive for influenza viruses, of which 1715 (70%) were typed as influenza A and 718 (30%) as influenza B. Of the sub-typed influenza A viruses, 101 (7%) were influenza A(H1N1)pdm09 and 1315 (93%) were influenza A(H3N2). Of the characterized B viruses, 66 (47%) belong to the B-Yamagata lineage and 74 (53%) to the B-Victoria lineage. (FluNet data as of 3 January 2012, 13:50 UTC)

Laboratories may update FluNet retrospectively as results become available. This leads to updates of previous summary data.